Several recording technologies have been used over the past years for recording data or other forms of information-bearing signals onto optical record members, such as rotatable disks. Ablative recording has been used for the so-called write once/read many (WORM) recording systems. The information recorded on the record member is optically sensed by shining a light beam onto the recorded area and sensing the intensity modulation in the reflected light. For ablative systems, the medium space between ablated areas provides for high reflection of light while the ablated areas reflect very little, if any, light into the sensing mechanism. The record member intensity modulates a read beam for enabling faithful recovery of the recorded information. In a similar manner, so-called phase-change materials record information by changing the phase of the record member between crystalline and amorphous states. The surface reflectivity of the two states is different for intensity modulating a read light beam. Bipolymer systems can also be used, which reflect or transmit light of varying colors with varying intensities. Yet another system for optical recording uses magnetooptic technology. During the recording process, a magnetic steering field envelopes the area to be recorded. A laser beam shines onto the recording area for heating a spot to be recorded above its Curie point temperature on the disk magnetooptic coating. Upon cooling, the heated area magnetically follows the magnetic steering field for recording the information represented by a modulated write beam from a laser. For reading or tracking through the recorded area, the laser light beam has a reduced intensity such that the record area is not heated above the Curie point. Generally, such magnetooptic (hereinafter MO) record members require erasure to a reference state before each recording operation, such as to a binary zero state (a so-called south pole being adjacent the recording and reading surface of the disk). Then, on a subsequent access to the record member, data can be recorded by selectively thermomagnetically switching portions of the record member such that the north pole terminates at the reading and recording surface of the disk. The magnetic steering field is reversed during erasure.
An unfortunate characteristic of many MO record members is that of media defects. Many of the defects can occur or be generated when the record member is first manufactured. Depending upon record member construction, additional defects can arise after the manufacture and even after data or other information-bearing signals are recorded on an MO record member. Further, the size of the initial defects resulting from the manufacturing process may change in size. As a result, powerful error detection and error correction code (ECC) systems are employed with the MO disk for accommodating such media defects. It is to be appreciated that the other media types are also subject to high defect rates. The effective defect rates in MO are often higher than other magnetic recording media because of the increased density (smaller size signal recording areas) than commonly employed with magnetic recorders. Many magnetic media are manufactured which use expensive, high quality manufacturing processes. It is desired to avoid such additional expense in the optical recording area, yet provide for a high level of data integrity during recording and readback operations using optical media. Such integrities include checking the media for new or old defects, and checking for residual data, i.e., whether or not the record area has been faithful erased for faithful MO recording and readback. Such checks also test the operativeness of the data signal channel electronics.